Female fish use their fins to attract males

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Biologists have described, for the first time, that it is not only males that develop ornamental fins to attract mates of the opposite sex.

Research published in the open access journal BMC Evolutionary Biology has shown that the male cichlid Pelvicachromis taeniatus, prefers females displaying a larger pelvic fin, and that this preference has driven females to grow pelvic fins that are out of proportion to their body size.

Previous to the research, it was widely accepted that fin size and ornamentation in female cichlids was considered important only for movement or egg placement, and had no social function. 

The research also suggests the evolutionary development of an oversized fin may not only serve a courtship purpose as a display of fecundity, but may also be used in battles between females; the possession of a larger pelvic fin may signal certain advantages held by a female when the females are sizing each other up.

Experiments were conducted in the lab using fish bred from wild populations of the Moliwe river in Cameroon, West Africa. 

To allow for experimental standardisation, computer animations were used to provide artificial stimuli to the males. The males associated more often with the female animations that displayed a larger pelvic fin, and the reasoning offered is that females with the larger pelvic fins are signalling to the male that they are of a higher quality i.e. genetically, or their ability to care for offspring, than females with a smaller pelvic fin.  

From an evolutionary standpoint, the study is interesting as it suggests that both sexual selection and natural selection may have played a part in the development of such ornamentation. 

Sexual selection has encouraged the growth of an elaborate pelvic fin, whereby natural selection has constrained the size of the fin to ensure an adequate ability to escape predators.

For more information, see the paper: Male mate choice scales female ornament allometry in a cichlid fish. Sebastian A Baldauf, Theo CM Bakker, Fabian Herder, Harald Kullmann and Timo Thunken.  BMC Evolutionary Biology 2010, 10:301doi:10.1186/1471-2148-10-301.

PFK covered a separate study back in 2007, in which Pelvicachromis taeniatus was found to have extraordinarily long sperm.